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SubjectRe: pci_set_mwi() ... why isn't it used more?
On Mon, Jan 20, 2003 at 10:41:35AM -0800, David Brownell wrote:
> I was looking at some new hardware and noticed that it's
> got explicit support for the PCI Memory Write and Invalidate
> command ... enabled (in part) under Linux by pci_set_mwi().
>
> However, very few Linux drivers use that routine. Given
> that it can lead to improved performance, and that devices
> don't have to implement that enable bit, I'm curious what
> the story is...
>
> - Just laziness or lack-of-education on the part of
> driver writers?
>
> - Iffy upport in motherboard chipsets or CPUs? If so,
> which ones?
>
> - Flakey support in PCI devices, so that enabling it
> leads to trouble?
>
> - Something else?
>
> - Combination of all the above?

You missed the reason entirely ;-)

pci_set_mwi() is brand new, I just added it. Hasn't filtered down to
drivers yet. The few drivers that cared prior to its addition, like
drivers/net/acenic.c, just hand-coded the workarounds needed for proper
MWI support on all chipsets.

pci_set_mwi() would not exist at all, were it not for the existing
hardware quirks. (if hardware were sane, drivers would just
individually twiddle the _INVALIDATE bit in PCI_COMMAND, and never call
functions other than pci_{read,write}_config_word.

Jeff



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